This turned out to be a client problem in that IE does not work properly with NPVR's web interface. Sub had me try the Edge browser and it works fine. Sub also told me that he has tested on Chrome. I'm going to install and try Firefox on the client and see how that works and will post back. Note that, in my case, both the "server" that runs NPVR and the client are Windows 10. I cannot comment on other environments.